Nitrogen Budget In Zhanjiang Area And The Flux Estimation Of Nitrogen In Zhanjiang Bay With South China Sea

Zhanjiang municipality located in southwest of Guangdong Provence, the low latitude area where south of the tropic of cancer. Affected by the monsoon and ocean,zhanjiang has obvious monsoon climate characteristics for high temperature, long summer, no winter, rich precipitation and small wind. Zhanjiang bay was surrounded by land, suixi river inflow from the top of the bay and some streams which flow through the urban area also inject the gulf. Zhanjiang bay has the ports, wharfs,anchorage and mariculture zone, communicate with open waters of northwest South China Sea through the mouth of the gulf. Therefore, the biogeochemical cycle and the balance of the material elements in the Zhanjiang bay are complex. This paper selected Zhanjiang area as the research region. From the perspective of nitrogen,nitrogen budget model was established, discussed the nitrogen sources and destinations, the evolution of income and expenditure, affecting factors and its environmental effects in Zhanjiang area in the context of China’s industrialization,urbanization and population growth, and used the LOICZ biogeochemical element budget model to study the exchange flux of nitrogen between Zhanjiang bay and open sea area under the influence of the small diameter flow stream. We wanted to explicit the nitrogen sources and sinks and cycle processes in Zhanjiang region. To provide a basis for scientific evaluation and control of nitrogen pollution in Zhanjiang area.The article research conclusions were as follows:From 2002 to 2013, the total nitrogen input of Zhanjiang area fluctuated during35 444.03-40 466.69 t. The contribution of nitrogen source followed as fertilizer nitrogen, human and animal excretion, atmospheric settlement, farmland, crop residual body and industrial wastewater. The nitrogen load of the past years was in the range of 210.10-277.17 kg·hm^-2·yr^-1, overall showed decline trend. Nitrogen load of Zhanjiang area was higher than that of the Beijiang river basin, the Pearl river delta region, lower than that of the Yangtze river delta region. Elevated nitrogen load increased the risk of nitrogen loss. The total amount of nitrogen output of Zhanjiang area in 2002-2006 was increased from 30 546.14 t to 35 541.68 t, which began todecline in 2007, and was stable after 2008. Each year the contribution of output pathways were relatively stable, and the largest amount of nitrogen output was water output, the second way was denitrification, ammonia volatilization and crop output was far less than the first two. The annual surplus of nitrogen was between 3 730.22 and 6 865.9 t, and the nitrogen surplus load was reduced from 47.03 kg·hm^-2·yr^-1 in2002 to 23.48 kg·hm^-2·yr^-1 in 2013. The surplus nitrogen was stored in the soil, water and vegetation of the area. With agriculture as the main industry structure, the amount of fertilizer nitrogen in the area of Zhanjiang area was high, and the ratio of harvest nitrogen/farmland fertilizer nitrogen was less than 40%. The potential nitrogen pollution and environmental risk brought about by the imbalance of nitrogen budget was besetting Zhanjiang region.The residual volume of water in the Zhanjiang Bay is supervised by the runoff,precipitation and evaporation. The dry season less precipitation, the stream flow into zhanjiang bay belongs to the type of rain flood, zhanjiang bay water is less than the output, offshore supplies are needed to maintain balance. In the rainy season input is greater than the output, water of zhanjiang bay flow to the coast. Drought season and rainy season in 2009, VR showed different characteristics. In dry season, water flows from the South China Sea to the Gulf of Zhanjiang. While the rainy season was opposite. Zhanjiang Bay system had the residual flux of-12.149×10⁸ m3 in 2009,which flowed from Zhanjiang Bay to the South China Sea. Water exchange flux VX was highly influenced by system salinity change, while VR was not affected basically.Coastal waters of Zhanjiang Bay were affected by the northeast monsoon and coastal water, which were constituted by the Pearl River runoff into the sea, Seawater salinity of Zhanjiang bay was influenced by external waters. Salinity between the inner and outer of Zhanjiang Bay was changed. VX values were approximately one order of magnitude greater than VR.DIN flux in 2009, exchange flux VXNX and residual flux VRNR in drought and rainy seasons were consistent. DIN of Zhanjiang bay to be off supplies to maintain balance in the dry season. While DIN was flowed from Zhanjiang Bay to the South China Sea in the rainy season. The nutrient concentration in the system would directly affect the nutrient flux,when the value of the runoff was close. The exchange flux of DIN and the residual flux of DIN was-2 708.21×10⁸ and-260.78×10⁸ mmol,respectively. DIN was flowed from Zhanjiang Bay to the South China Sea in 2009.The difference of DIN total amount between output and input was ΔDIN, whose valuewas-59.02×10⁸, 864.77×10⁸, 109.36×10⁸ and-31.53×10⁸ mmol in 2, 5, 8, 11 month of 2009. Zhanjiang Bay was the source of DIN in rainy season, and dry season was opposite.ΔDIN=2 192.71×10⁸ mmol in 2009, Zhanjiang Bay also was the source of DIN.In the process of nitrogen budget of Zhanjiang in 2009, the amount of nitrogen through the water and precipitation output pathway transported into the Zhanjiang Bay system was 11 890.22 t, the amount of nitrogen inflowed to South China Sea from Zhanjiang Bay system was 3 069.79 t,eventually. The ratio of nitrogen output /input in Zhanjiang Bay was 25.82%. Most of the input nitrogen was reserved in the system of Zhanjiang Bay by nitrogen fixation and sedimentation.Oct. 2015 and Jan. 2016, VX and VR showed a consistent pattern within 2009.The exchange flux of DIN was-190.60×10⁸ mmol and the residual flux of DIN was-75.57×10⁸ mmol in Oct. 2015, while the value was-62.54×10⁸ mmol and-286.50×10⁸ mmol in Jan. 2016, respectively. Both of the two months, DIN was flowed out of the system area. ΔDIN=152.99×10⁸ mmol（2015.10） and 273.30×10⁸mmol（2016.01）, were positive. Zhanjiang Bay was the source of DIN in months of rainy season. In months of dry season, Zhanjiang bay showed the sink of DIN in Feb.and Nov. 2009 exchanged into the source of DIN in Jan. 2016.